Who are the strikers?

The RMT union, headed by the leftwinger Bob Crow, is the largest of four unions involved in tube staff pay negotiations with Transport for London (TfL) and the only one to take strike action. The others are the train drivers' union Aslef, Unite and the Transport Salaried Staffs' Association (TSSA).

Why are they striking?

It has some of its roots in the collapse of Metronet, the infrastructure giant that collapsed and was brought back under public control last year, leading London Underground (LU) – part of TfL – and the London mayor, Boris Johnson, to insist that 1,000 jobs must go because of duplication. TfL inherited 7,000 staff from Metronet and says this has led to the duplication of many roles, mainly in administration, legal and finance departments, as well as among contractors.

The RMT is pressing for a guarantee of no compulsory redundancies despite assurances from TfL that it aims to achieve the cuts through voluntary means and that no frontline staff such as train drivers, station staff or engineers will lose their jobs. Union leaders have accused TfL of being prepared to tear up agreements over job protection, arguing that tube workers who were transferred to Metronet under the controversial public-private partnership were given a continuity of employment contract.

The union believes the document is in effect a no compulsory redundancy agreement, protecting staff who are once again employed by LU or TfL – a view not shared by TfL. The second issue is over pay, although negotiators believed the two sides were very close to reaching agreement after LU tabled two revised offers when an initial five-year deal was rejected.

The union has complained of a culture of bullying and harassment at LU, and the failure to follow the agreed disciplinary procedures. The case of two members who were sacked crept into the talks on Monday, with each side blaming the other for unhelpfully adding this to the mix. One of the sackings, over an incident linked to safety, has already led to strikes on the Victoria line.

What is the current pay offer?

The new offer is either a two-year deal worth 1% this year and the rate of inflation plus 0.5% next year; or a four-year deal of 1.5% now and the rate of inflation plus 0.5% in future years.

Is pay generally poor among tube workers?

It depends what you compare it with. A station supervisor earns £35,000-£39,000, while a tube driver's starting salary is just over £40,000, according to TfL. Pay grades for station assistants start at either £24,000 or £29,000 depending on the role and responsibilities.

What happened to the negotiations?

Ten-hour talks broke down on Monday. London Underground and RMT negotiators were then invited to conciliation by the Advisory, Conciliation and Arbitration Service (Acas) the next day. Progress was made when pay was removed from the talks following an agreement by the RMT to take the two pay offers to members to see what they think.

The fiddly issue of the two sacked workers was also put to one side. The outstanding issue was job protection. The RMT believed an agreement was made on the wording, whereas TfL said it had only considered the wording put forward by the RMT and after taking advice, decided more talks were necessary. At this stage, with minutes to go before the strike deadline of 6.59pm, talks broke down, with both sides blaming the other for the walkout. The strike duly began.

Why are some tube lines still running?

This is because the RMT is not the only union representing tube drivers. Aslef did not ballot members and therefore have to continue working under strict laws in place. But it became clear that the Aslef leadership did not support the RMT decision to strike at this stage.

As for the RMT, its membership is traditionally loyal when it comes to industrial action. A ballot of London Underground staff was sent out to 10,000 RMT members, of whom 3,200 responded– 2,800 in favour of a strike and 400 opposed. The RMT said this relatively low turnout was because it was forced to ballot twice after TfL quibbled with the first ballot.

What happens next?

The RMT is incensed that the London mayor and chair of TfL, Boris Johnson, has not joined the negotiations. The union points to his failure to meet the RMT despite his pledge 16 months ago to secure a no-strike deal with tube unions.

Johnson has called the RMT "demented" and accused it of "taking the piss" out of Londoners with the strike, while his transport commissioner, Peter Hendy, has urged the RMT to come back to Acas and continue where things left off on Tuesday, and the union is reported to be considering this.

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Millions of passengers had another difficult commute into work as the RMT's 48-hour strike over pay and jobs on the London Underground continues. Follow live updates on the strike and the disruption, and share stories of your